In the printed circuit board (PCB) industry, it is necessary that the thin soft and pliable smooth and shiny clean copper foil be protected as by an adhered but releasable masking film or layer, from the depositing of airborne particulates and from marring; and that it also be buffered from imperfect separator plates and the like. The foil, furthermore, is preferably somewhat stiffened and somewhat cushioned to withstand transit, slitting, sheeting and tooling processes, and specifically, the handling during the PCB processing. The adhered protective layer or film, moreover, for certain processing purposes, must withstand high temperatures up to 400° F. and endure up to 700 psi of high laminating pressure, but then still be readily strippable from the copper foil to enable etching and photolithographic and other PCB surface processing. All this, moreover, with a critical film release from the copper foil that leaves absolutely no residue on the pristine shiny smooth foil and no marking or pattern embossing effect on the foil surface.
Heretofore, approaches to solve the above problem have involved the use of preferably polyester and other plastic layers, adhesively attached to the foil as in our prior U.S. Pat. No. 4,753,847, marketed under the trademark PACOTHANE. Adhesives, however, always raise the problems of cost, curing time, surface uniformity and residues, among others. Other approaches to the adherence of protective layers to copper foils or the like, however, are also described in our later U.S. Pat. Nos. 5,948,526 and 5,989,377. The Sentrex Company has offered a product under the mark COPPERGARD using a 3-mil low-density polypropylene with 1.25-mil linear low-density polypropylene attached on both sides of the copper foil, and has obtained U.S. Pat. No. 6,280,851. A further U.S. Pat. No. 5,120,590 of Gould Inc. has also proposed protective layers for an electrodeposited metal foil; and a Mitsui Mining U.S. Pat. No. 5,942,314 discloses ultrasonically weld-bonding of aluminum to the copper. The REServices Company in its product SC, spot-adheres stainless steel to the copper.
The present invention was stimulated by questioning and exploring whether suitable protective layers could be releasably attached to the copper foil without resorting to the use of conventional physical adhering techniques—adhesives, bonding, etc.—but using instead the temporary adhesion of permanent electrical charges. While charge-adhesion had been long used for other purposes including more recently primarily in the graphic arts industry as for providing compositor printing sheets comprising a backing and a printing sheet including a plastic electret sheet subjected to electrostatic fields under high voltage corona arc discharge, as disclosed, for example, in the Permacharge Corporation U.S. Pat. No. 6,284,339 and also in U.S. Pat Nos. 5,989,685 and 5,904,985, such, however, involve very different products, problems, requirements and processing than those of the printed circuit board field and of thin metal foils and the like, so that the electrical charge adhesion concept did not at first seem applicable to the radically different products, needs and conditions of PCB manufacture. Questions arose, for example, as to whether an electric-charge adherence could be strong enough; how long can the charge last; what does humidity do to the charge; would the charge deleteriously affect the protective layer release performance; would the charge impact the copper foil in the laminating process; and would the charge hold during the rigorous handling and processing conditions of PCB operations; etc.? Our early attempts, indeed, to use the electric-charge process with its conventional corona or arc treatment as used in the graphic arts industry for its purposes, indeed, failed to meet the PCB requirements. Modification of the conventional electric-charge approach, including elimination of corona treatment and the use preferably of select polyester plastic protective layers and the like, improved charge adherence to, for example, 232 grams of force in one test, and ultimately led to the successful adaptation and use of this charge-adhesive technique for purposes of the present invention.